The present invention relates to a charged beam apparatus such as a synchrotron or a storage ring which accelerates a charged beam such as an electron beam, stores the accelerated beam and utilizes the synchrotron radiation that is generated at the beam bending portions. More particularly, the present invention relates to an improvement of a cryogenic vessel for a charged-beam deflection superconducting magnet (this vessel is hereinafter sometimes referred to as a cryostat), as well as to a technique for shielding leakage flux and correcting the distribution of deflection magnetic field.
FIG. 1 shows schematically the operating principles of a storage ring 100. In the Figure, reference numeral 1 designates a vacuum chamber for providing a passage for a charged beam, 2 a vacuum chamber for guiding synchrotron radiation, 3 a deflection magnet for bending the charged beam, 4 synchrotron radiation, 5 a vacuum chamber for guiding the charged beam into the storage ring, and 6 the charged beam. The apparatus and components that do not have any direct relation to the present invention are not shown in FIG. 1.
In the actual apparatus, the vacuum chamber 1 for charged beam passing through the deflection magnet 3 is provided with a plurality of vacuum chambers 2 for synchrotron radiation that are slightly staggered in position one to another. For the sake of clarity, FIG. 1 shows the use of a single vacuum chamber 2 for one deflection magnet 3.
The operation of the storage ring 100 will proceed as follows. A charged beam (typically an electron beam) 6 accelerated close to the velocity of light is injected into the storage ring 100, and the beam travels through a circle of vacuum chambers 1 as it is deflected by deflection magnets 3. When the beam 6 is deflected by a deflection magnet 3, synchrotron radiation 4 is generated in a direction tangential to the beam's orbit. This radiation has a broad spectrum ranging from soft X-rays to visible light and provides a superior radiation source.
The intensity of synchrotron radiation 4 is proportional to the charged beam current which in turn is proportional to the quantity of charged beam in the storage ring. In order to increase the charged beam current, the pressure in the vacuum chambers for charged beam, which are connected to the vacuuum chambers for synchrotron radiation, must be reduced to an extreme high vacuum, which typically is within the range of 10.sup.-9 to 10.sup.-10 Torr. An ultrahigh vacuum of the same order is also required for prolonging the life time of charged beam. If a sufficient vacuum is not produced, the charged beam will collide with the gas molecules or ions in the vacuum chambers to attenuate the charged beam current. As a result, neither the charged beam current nor the life time of the charged beam can be increased; in other words, synchrotron radiation of high intensity cannot be produced for a long period of time.
FIG. 2 shows a typical coil winding for a superconducting magnet used as a deflection magnet, with the direction of current flow being indicated by the arrows. The coils shown in FIG. 2 are placed in a cryostat to make a superconducting deflection magnet. FIG. 3 shows in cross section the construction of a conventional superconducting deflection magnet such as the one described in "IEEE TRANSACTION OF MAGNETICS", vol. MAG-15, No. 1, JAN., 1979, pp. 131-133.
In FIG. 3, reference number 31 designates a superconducting coil, 32 a coil support structure, 33 liquid helium for cooling the coil 31, 34 a helium container (vacuum-resistant), 35 a heat insulating vacuum space (which typically is evacuated to a pressure of about 10.sup.-6 Torr), 36 heat shielding liquid nitrogen, 37 a nitrogen container (also vacuum-resistant), and 38 a vacuum vessel. A vacuum chamber 1 for charged beam also serves as an inner vacuum vessel for the magnet. An example of the direction of a deflecting magnetic field for deflecting the charged beam is indicated by the arrow. Although not shown, a spacer for retaining a gap is disposed between individual structural components.
An application of the conventional superconducting deflection magnet to a storage ring is shown schematically in FIG. 4, wherein synchrotron radiation is extracted through a vacuum chamber 2 provided on the side of the vacuum vessel 38. The vacuum chamber 2 which also serves as the inner vacuum chamber for the magnet is connected to the vacuum vessel 38 in a vacuum-resistant manner. In the case shown, the superconducting coil is divided into two sections, upper and lower, which are sufficiently spaced from each other to accommodate the vacuum chamber 2 for synchrotron radiation.
Having the construction described above, the conventional superconducting deflection magnet has one major problem: the ultrahigh vacuum (10.sup.-9 to 10.sup.-10 Torr) in the vacuum chambers for charged beam and synchrotron radiation are connected to the heat-insulating vacuum (about 10.sup.-6 Torr) in the cryostat by the same vacuum wall (i.e., vacuum chambers 1 and 2 in FIGS. 3 and 4), so if the ultrahigh vacuum is deteriorated and it becomes necessary to repair or replace the vacuum chamber for charged beam or synchrotron radiation, the cryostat must also be disassembled. Since high level techniques are required to attain an ultrahigh vacuum in the range of 10.sup.-9 to 10.sup.-10 Torr, the probability that the ultrahigh vacuum will be deteriorated during operation of the storage ring would be considerably higher than the probability of deterioration in the heat-insulating vacuum in the cyostat.
FIG. 5 shows a charged beam apparatus of the type described in "Superconducting Racetrack Electron Storage Ring and Coexistent Injector Microtron for Synchrotron Radiation", by T. Miyahara, K. Takata and T. Nakanishi, TECHNICAL REPORT of ISSP, Ser B No. 21, 1984. In the Figure, reference numeral 51 designates a septum magnet for injecting charged particles into a storage ring, 52 superconducting coils forming a superconducting magnet, 53 an iron yoke, 54 a quadrupole magnet, 55 kicker magnet, 56 radio-frequency cavity, 57 a sectupole magnet, 58 a monitor, 59 an octupole magnet, 60 a vacuum pump, 61 a synchrotron radiation port, and 64 a vacuum chamber.
The operation of the apparatus shown in FIG. 5 will be described. Charged particles accelerated to a sufficient speed are bent with the septum magnet 51 and guided into the ring. The injected particles are then adjusted with the quadrupole magnet 54, sextupole magnet 57 and octupole magnet 59 to keep moving along predetermined orbits. When the travelling direction of the particles is bent with the magnetic field of the superconducting coils 52, synchrotron radiation is produced in a direction tangential to the orbit of the particles. The energy lost by emission of the synchrotron radiation is compensated in the radio-frequency cavity 56 and this provides sufficient energy for the charged particles to continuously travel through the ring. The emitted synchrotron radiation is guided to the outside by way of the synchrotron radiation port 61 and utilized as a radiation source.
The superconducting coils 52 used in the apparatus of FIG. 5 have a uniform and very strong magnetic field of about 4[T]. In comparison, the quadrupole magnet 54, sextupole magnet 57 and octupole magnet 59 have weak fields of about 1.4[T]. The deflection radius .rho., the energy of charged particles E, and the deflection field B created by the superconducting coils 52 can be correlated by B=E/0.3 .rho.. If, for instance, the stored energy E is increased with a view to extracting more intense synchrotron radiation, or if .rho. is decreased in order to reduce the overall size of the equipment, B will increase progressively to such an extent that the necessary amount of B cannot be supplied by a normal conducting magnet and can only be attained by the superconducting coils 52. However, the strong field of the superconducting magnet causes magnetic saturation or nonuniformity of magnetic field, which leads to an increased leakage flux at the ends of the coils. The excessive leakage flux will either impair the fields of nearby magnets or impart an unwanted magnetic field to the charged particles.
In short, the conventional charged beam apparatus having the construction described above has the following problems: if the deflecting magnetic field is increased, more leakage flux will occur to impair the uniformity of the fields of magnets located in the neighborhood of the deflection magnet; in addition, the charged particles travelling on predetermined orbits are subjected to the action of unwanted fields and become unstable within the ring, and they will thus vanish as a result of collision against the ring wall. The problem of leakage flux will become more pronounced if a progressively stronger deflecting field is required in such cases as when one wants to obtain strong radiation or reduce the overall size of the equipment.
FIGS. 6(a) and 6(b) show still another example of the conventional charged beam apparatus. An ultrahigh vacuum chamber 71 through which a charged beam travels and which is evacuated to a pressure of the order of 10.sup.-9 Torr (this chamber is hereinafter referred to simply as a vacuum chamber) consists of a plurality of straight sections 71a in which the charged beam travels on a straight line and an equal number of sections 71b in which the beam is deflected. A deflection electromagnet 72 is formed of superconducting deflection coils 73 (which are hereinafter referred to as superconducting coils) and disposed in each of the deflecting sections 71b. An equilibrium orbit 74 for the charged beam is formed within the vacuum chamber 71. A charged beam region 75 represents the area of spatial location where the charged beam exists. The charged beam is injected into the system at entrance 76.
The operation of the system of FIG. 6 will be described. After being injected into the vacuum chamber 71 through entrance 76, the charged beam will keep moving along the predetermined orbit 74 formed by the deflection electromagnet 72. If the system is used as an electron storage ring, the charged beam will produce synchrotron radiation when its orbit is bent and the resulting radiation is extracted for further use. A cross section of the beam in the vacuum chamber 71 has a certain amount of spread to form the charged beam region 75. In other words, the charged beam consists of particles that continue to move on the orbit 74 while experiencing small oscillations. It is therefore necessary to impart a predetermined deflecting magnetic field to the entire part of the charged beam region. If the beam is subjected to varying amounts of deflecting field in different positions of the beam region 75, it becomes impossible to confine the beam in the region 75 and the charged particles will collide against the wall of the vacuum chamber 71 to gradually lose their energy. Various efforts and proposals have therefore been made in order to provide a deflecting magnetic field having a maximum degree of uniformity throughout the charged beam region 75.
If the deflection electromagnet is composed of normal conducting coils, the use of an iron yoke will provide a uniform field fairly easily. On the other hand, the use of superconducting coils 73 has been proposed with a view to producing a stronger magnetic field and achieving reduction in the overall size of equipment. However, if an iron yoke is used with superconducting coils it must be accommodated in a cryostat and problems will occur in association with heat load and support mechanism, which leads to an increase in the overall size of the deflection magnet or in the cooling cost. If no iron yoke is used as in the conventional case shown in FIGS. 6(a) and 6(b), the deflection magnet is not capable of producing a uniform field in the radial direction of the beam region 75 and will suffer from unwanted beam accumulation and reduced beam life time.